Evaluation and Optimization of Municipal Sewage Sludge Vermicompost Process as an Effective Option for Organic Waste Recycling
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Date
2021-07-27
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Addis Ababa University
Abstract
Population increases and unplanned urbanization in urban cities like Addis Ababa have led to an increase in wastewater generation and sludge production. The management of sludge is among the most costly and challenging processes of any WWTP bearing the fact that disposal options should protect public health and the environment. This study, therefore, employed an ecological cheap, and natural way of organic waste recycling for agricultural practices using the vermicomposting approach. Samples of wet and dry municipal sludges were obtained from the outlet of the USAB and sludge drying beds of Kaliti WWTP to characterize the municipal sludges. Vermicompost was produced from the dry sludge samples amended with cow dung and coffee husks to balance the carbon to nitrogen ratios of the organic waste for faster degradation. Optimization of the process parameters was carried out using Design Expert Version 10.0.7 under an RSM and Box-Behnken Design. The three parameters and their ranges namely moisture content (60-90%), turning frequency (1-3 turnings/week), and mixing ratio of substrates (50:50 to 80: 20 wt.%) of municipal sewage sludge to amendments, respectively were chosen. The experimental data on the quantity of N, P, and K were fitted into a quadratic polynomial model using multiple regression analysis. The regression coefficients of 99.91%, 89.65%, and 99.46% for NPK observed between the predicted and actual values were obvious that the developed regression models fitted the experimental data well. A regression of coefficient of 0.75 is an indication of the aptness or suitability of the model. The experimental results show that the moisture content of 71.95%, mixing ratio of sludge to amendments (72.3: 27.7% wt.), and a turning frequency of 2 turnings per week were the best operating conditions. Under these conditions, the maximum calculated yield of N, P, and K was 2.76%, 1.80%, and 1.88% respectively. A plastic pot experiment was, also, set up to evaluate the efficiency of the vermicompost to enhance plant growth using a germination index value. The study was conducted through the effect of increasing concentration of vermicompost (T1- 0% VC, T2 - 50% VC, and T3 -100% VC w/w) in the target germination rate or index. Results showed that the germination index produced from T3 was > 33 times T2 and > 66 times T1. It was inferred that vermicomposting of municipal sewage sludge is an effective option for recycling nutrients in organic waste.
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Optimization, Vermicomposting, Municipal Sewage Sludge, Eisenia Fetida, Coffee Husks, and Cow Dung